1. Field of the Invention
This invention relates to light detecting apparatus to be used in optical communication and so forth. The light detecting apparatus detects or monitors wavelength fluctuations of a received light and the detected information is used for a wavelength tracking in a filtering or demultiplexing portion of the apparatus, or to control an oscillation wavelength and/or output power of a laser device coupled to the light detecting apparatus. The light detecting apparatus may preferably be used in a transmitter or receiver end office of an optical communication network or system.
2. Related Background Art
Conventionally, a wavelength band width per one signal wavelength is set considerably wide, such as 20 nm, in a wavelength division multiplexing communication system in which a plurality of signal wavelengths are distributed over a band arround 0.8 .mu.m, is consideration of the performance of a filter disposed at the side of a signal receiver. Therefore, even if the wavelength of a received light fluctuates to some extent in such wavelength band, a signal can be received well at the receiver side and there is no obstacle to the wavelength division multiplexing communication. As a result, a wavelength tracking operation is not needed for each signal wavelength at the receiver side.
In such a prior art example, however, it is difficult to increase the multiplexing degree or multiplicity of signal wavelengths since the band width per one signal wavelength is set wide, and hence the advantage of wavelength division multiplexing communication is not sufficiently utilized although the wavelength tracking operation is dispensable.